1. Field of the Invention
The present invention relates to array antenna radio communication apparatuses used in radio communication systems.
2. Description of the Related Art
An array antenna includes a plurality of antennas and is capable of freely setting reception directivity by adjusting the amplitude and phase of signals received from respective antennas. Adjustments to the amplitude and phase of a received signal can be carried out by multiplying the received signal by a complex coefficient in a received signal processing section.
FIG. 1 is a block diagram showing the configuration of a radio communication apparatus equipped with array antennas. FIG. 1 shows an example of communication apparatus with two antenna devices.
When communicating with another communication apparatus, this communication apparatus operates as follows. Radio signals are received through reception antennas 4 and 5. The received radio signals are supplied to reception radio circuits 8 and 9 via switching sections 6 and 7. As the switching sections here, various means can be used such as cable switching, mechanical switches and electronic switches. The received radio signals are down-converted to base frequency band or intermediate frequency band signals in reception radio circuits 8 and 9 and supplied to received signal processing section 10. Inside received signal processing section 10, demodulation processing is performed. The configuration of received signal processing section 10 is determined accordingly by the communication system used.
It is possible to selectively receive a certain electromagnetic wave coming from a desired direction with stronger power than other waves by adjusting a complex coefficient to be multiplied inside received signal processing section 10 above. This is called xe2x80x9cbearing reception directivity.xe2x80x9d By bearing directivity it is possible to keep a reception SIR (Signal to Interference Ratio) high.
However, the characteristics of reception radio circuits 8 and 9 vary depending on the circuit because of variations in the characteristics of analog devices such as amplifiers. This adds to the received signal of each antenna unknown different amplitude variations and phase rotations, resulting in the formation of reception directivity different from the expected reception directivity obtained by multiplying a complex coefficient in received signal processing section 10.
In order to prevent such a phenomenon, adjustments need to be made so that the characteristics of reception radio circuits 8 and 9 may be identical. However, it is extremely difficult to adjust the characteristics of analog devices such as amplifiers accurately and in a time-invariable manner. Therefore, instead of adjusting the characteristics of reception radio circuits 8 and 9, a certain method is adopted by which the characteristics of reception radio circuits 8 and 9 are measured and stored in memory beforehand and a complex coefficient multiplied in received signal processing section 10 is determined by taking into account the fact that the amplitude and phase of the received signal change by the difference in their characteristics. Such an adjustment process is called xe2x80x9ccalibration.xe2x80x9d
Calibration is carried out before starting communications to measure the characteristics of the reception radio circuits. The following is an explanation of the calibration method.
A calibration signal is generated using calibration signal generator 1. Then, through power control section 2 such as an attenuator, the power of the calibration signal is controlled. The power-controlled calibration signal above is then distributed by distribution section 3, supplied to reception radio circuits 8 and 9 via switching sections 6 and 7. Here, distribution section 3 can be implemented using a distributor capable of supplying two-or more signals or switches that supply only one signal or cable switching.
The received signals of the reception radio circuits are observed by received signal processing section 10 and deviations from the expected amplitude and phase of the output signals of reception radio circuits 8 and 9 are stored in a calibration table as the characteristic differences to be corrected at the time of communications. Since the characteristic differences are measured for each reception radio circuit independently, calibration tables are also created independently by the number of reception radio circuits. The calibration tables are incorporated in recording section 11 provided inside or outside received signal processing section 10.
To observe differences in the reception characteristics due to the differences in the power of received signals, their amplitudes are changed by power control section 2 and the same processing is carried out. If distribution section 3 provides only one output at a time, processing is repeated by the number of antenna branches of this communication apparatus. If distribution section 3 supplies a plurality of outputs, calibrations corresponding to a plurality of antenna branches can be carried out simultaneously.
Through the processing above, reception calibrations for all antenna branches are completed. Then, the inputs of the reception radio circuits are switched by the switching sections to the reception antennas and communications are started. The received signal processing section carries out processing during communications with reference to the calibration tables so that the recorded characteristic differences of the reception radio circuits may be offset.
However, the conventional array antenna communication apparatus above has problems as shown below.
In order to observe the differences in the reception characteristics due to the differences in the power of received signals, their amplitudes must be changed by the power control section. However, in the power control section such as an attenuator and variable gain amplifier, controlling the amplitudes may affect signal propagation delay times, causing unexpected phase rotations to be added to the received signals. The phase characteristics of the reception radio circuits measured here result in a combination of phase rotations produced by the reception radio circuits themselves and those produced by the power control section, causing erroneous characteristics to be stored in the calibration tables. This will cause erroneous corrections to be made to the received signals during a communication, preventing correct formation of reception directivity.
It is an objective of the present invention to provide an array antenna radio communication apparatus capable of obtaining accurate reception directivity even if the power of a received signal varies.
This objective is achieved by an array antenna radio communication apparatus comprising two calibration signal generators; a calibration desired signal generator and calibration interference signal generator, which controls only the output of the calibration interference signal generator through a power control section and combines this power-controlled calibration interference signal and the calibration desired signal with fixed power into a combined calibration signal using a combination section.
When changing the power of the combined calibration signal in this apparatus, the power of the calibration desired signal is fixed by the power control section to avoid phase rotations and only the power of the calibration interference signal is changed by the power control section.
This combined calibration signal is supplied to a plurality of radio circuits simultaneously or alternately and reception processing is applied only to the desired signal in the received signal processing section and its reception characteristics are measured.
Through such a configuration and operation, the phase of the measured calibration desired received signal will no longer include phase rotations produced by the power control section. This makes it possible to measure the reception characteristics correctly when the power of the received signal varies, create accurate calibration tables and obtain accurate reception directivity using those calibration tables.